> On 25 Apr 2025, at 14:51, Vladimir Homutov via nginx-devel > <nginx-devel@nginx.org> wrote: > > On Tue, Apr 15, 2025 at 03:02:02PM +0000, nore...@nginx.com wrote: >> details: >> https://github.com/nginx/nginx/commit/cd5e4fa1446dff86fafc3b6ffcc11afd527a024f >> branches: master >> commit: cd5e4fa1446dff86fafc3b6ffcc11afd527a024f >> user: Roman Arutyunyan <a...@nginx.com> >> date: Sat, 4 Jan 2025 18:03:46 +0400 >> description: >> QUIC: do not increase underutilized congestion window. >> >> As per RFC 9002, Section 7.8, congestion window should not be increased >> when it's underutilized. >> > > Hello, > > it looks like this patch triggers some issues with ack processing > (possible previously, but now more probable): see details in suggested > patch. > > I'm also attaching bad.log and good.log. The first one demonstrates that > nginx is not sending any ACKs since it is switched into idle mode and > the window is not increasing. Since the client code is symmetric, deadlock > occurs.
cd5e4fa14 is not the only one to blame uncovering this apparently old bug. Few more things to consider: - 53e7e9eb5 initial congestion window reduction made this easier to trigger - a40cc7002 to ignore congestion in MTU probes made this easier to trigger The fix is to teach ngx_quic_output() to send ACKs on congested links. This meets RFC 9002, 7 and B.2 : An endpoint MUST NOT send a packet if it would cause bytes_in_flight : (see Appendix B.2) to be larger than the congestion window : bytes_in_flight: : Packets only containing ACK frames do not count toward : bytes_in_flight to ensure congestion control does not impede : congestion feedback. A simple reproducer attached.
h3_congestion_ack.t
Description: Binary data
-- Sergey Kandaurov
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